1. Field of the Invention
The present invention relates to a lithium secondary battery and electrodes therefor and a method of forming the same. More particularly, the present invention relates to a lithium secondary battery which prevents dendritic deposition of lithium metal from occurring due to repeated charge and discharge and electrodes therefor and a method of forming the same.
2. Description of the Related Art
In recent years, the amount of CO.sub.2 gas contained in atmosphere has been increasing, and it has been thus predicted that global warming occurs due to the greenhouse effect of CO.sub.2 gas. It is therefore difficult to newly construct a thermal power plant which converts energy generated by burning fossil fuel such as petroleum or coal into electrical energy and which discharges a great amount of CO.sub.2 gas.
In order to perform thermal power generation with a highest efficiency, it is preferable to operate the plant under constant conditions. Since the generated energy cannot be rapidly changed, the generated energy is controlled to providing power consumption in the daytime during which factories are generally consume a great amount of power. Under the present conditions, therefore, the generated power is wasted in the night during which power consumption decreases. It is thus proposed that, in order to effectively employ the electrical power generated by generators in a thermal power plant, the power generated at nighttime stored in secondary batteries installed in general homes, and is used in the daytime when the power consumption is increased, for leveling loads. This is known as load leveling. It is thus eagerly desired to develop secondary batteries which can be used for the load leveling.
It is also expected to develop high-energy density secondary batteries with excellent utility for electric vehicles which discharge no air pollutants such as carbon oxides (COx), nitrogen oxides (NOx), hydrocarbon (CH) and particles during driving.
Further, it is of urgent necessity to develop smaller secondary batteries which are more lighter weight and have higher performance, and which are used as power supplies for portable apparatus such as book-type personal computers, word processors, video cameras and portable telephones.
Since the reversible electrochemical lithium-graphite intercalation reaction was reported in JOURNAL OF THE ELECTROCHEMICAL SOCIETY 117,222 (1970), development of rocking chair type secondary batteries, i.e., "lithium ion batteries", as an example of small, lightweight and high-performance secondary batteries has proceeded in which carbon and an intercalation compound containing lithium ions are used as an anode active material and a cathode active material, respectively, so that lithium is intercalated into the carbon layer and stored therein by charge reaction. Some of such lithium ion batteries are being brought into practical use. In these lithium ion batteries, carbon serving as a host material for intercalating lithium as a guest material into the layer is used as the anode material so as to inhibit the growth of lithium dendrite during charge, thereby achieving long life in a charge-discharge cycle.
Since a secondary battery having long life can be achieved, as described above, the application of various types of carbon to anodes has been extensively proposed and investigated. A secondary battery is proposed in U.S. Pat. No. 4,702,977 in which a carbonaceous material having a hydrogen/carbon atomic ratio of less than 0.15, a (002) facing of 0.337 nanometer or more, and a crystallite c-axis size of 15 nanometer or less is used for an anode, and alkali metal ions such as lithium ions are used. Another secondary battery is proposed in U.S. Pat. No. 4,959,281 (Japanese Patent Laid-Open No. 2-66856) in which a carbonaceous material having a (002) facing of 0.370 nanometer or more, a true density of less than 1.70 g/cm.sup.3 and no exothermic peak at 700.degree. C. or more in differential thermal analysis in an air stream is used for an anode.
Examples of various types of carbon which are being investigated for use as battery anodes include carbon fibers (Denkikagaku, Vol. 57, p614 (1989)), mesophase microbeads (Extended Abstracts of The 34th Battery Symposium in Japan, p77 (1993)), natural graphite (Extended Abstracts of The 33th Battery Symposium in Japan, p217 (1992)), graphite whiskers (Extended Abstracts of The 34th Battery Symposium in Japan, p7 (1993)), and the burning product of furfuryl alcohol resin (Extended Abstracts of The 58th Annual Meeting of the Electrochemical Society of Japan, p158 (1991)).
However, a lithium ion battery which uses carbonaceous material as an anode active material for storing lithium and which has a discharge capacity exceeding the theoretical capacity of a graphite intercalation compound for storing one lithium atom for six carbon atoms, which permits stable discharge after repetition of charge and discharge, has not been yet obtained. The lithium ion battery using carbonaceous material as an anode active material thus has a long cycle life and a battery energy density lower than that of a lithium battery which uses a lithium metal as an anode active material.
It is difficult to bring a high-capacity lithium storage battery which uses a lithium metal for an anode into practical use because of the difficulties in preventing the occurrence of lithium dendrite, which mainly causes internal shorts, due to repetition of charge and discharge. When an anode and a cathode are short-circuited by the growth of lithium dendrite, heat is generated by consumption of the energy of the battery in the short-circuited portion within a short time, and gas is thus generated by decomposition of a solvent of an electrolyte. This causes an increase in the inner pressure of the battery and thus accidental damage to the battery.
A method has been proposed as a measure against this in which a lithium alloy such as lithium-aluminum is used for an anode for suppressing the reactivity of lithium. However, this method exhibits an unsatisfactory cycle life, and has not been brought into extensive practical use.
On the other hand, a high-energy density lithium secondary battery which has an energy density lower than that of a lithium primary battery and which uses an aluminum foil having an etched surface as an anode is reported in JOURNAL OF APPLIED ELECTROCHEMISTRY 22, 620-627 (1992). However, when a cycle of charge and discharge is repeated up to a practical range, cracks occur in the aluminum foil due to repeated expansion and contraction, thereby deteriorating the current collecting properties and causing the growth of dendrite. Thus, a secondary battery having a cycle life which permits use at a practical level cannot be obtained.
Therefore, it is eagerly desired to develop an anode material which has a long life and an energy density higher than that of a carbonaceous anode material which is currently put into practical use.
It is also necessary for realizing a high-energy density secondary battery to develop a cathode material. Under the present conditions, a lithium-transition metal oxide is mainly used as a cathode active material. However, the discharge capacity attained is only 40 to 60% of the theoretical capacity.
For lithium secondary batteries including "lithium ion batteries" which use lithium ions as a guest in charge-discharge reaction, it is strongly desired to develop an anode and a cathode which have a cycle life within a practical range, and capacity higher than that of an anode comprising carbonaceous material and a cathode comprising transition metal oxide, which are presently brought into practical use.